User equipment (UE) having an adaptive RF amplifier prelimiter

ABSTRACT

A user equipment (UE) for transmitting signals employing a CDMA technique comprises means for combining a plurality of spread spectrum data signals; means for measuring a characteristic of the output of the combining means for a given time period; and means for adaptively limiting an output of the combining means responsive at least partially to an output of the measuring means.

[0001] This application is a continuation of application Ser. No.09/386,876, filed Aug. 31, 1999, which application is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention generally relates to spread spectrum code divisionmultiple access (CDMA) communication systems. More particularly, thepresent invention relates to a system and method for adaptively limitingforward and reverse link transmission power within CDMA communicationsystems.

[0004] 2. Description of the Prior Art

[0005] Wireless communication systems using spread spectrum modulationtechniques represent the state of the art in digital communications andare increasing in popularity. In code division multiple access (CDMA)systems, data is transmitted using a wide bandwidth (spread spectrum) bymodulating the data with a pseudo random chip code sequence. Theadvantage gained is that CDMA systems are more resistant to signaldistortion and interfering frequencies in the transmission channel thancommunication systems using other multiple access techniques such astime division multiple access (TDMA) or frequency division multipleaccess (FDMA).

[0006] One indicator used to measure the performance of a communicationsystem is the signal-to-noise ratio (SNR). At the receiver, themagnitude of the desired received signal is compared to the magnitude ofthe received noise. The data within a transmitted signal received with ahigh SNR is readily recovered at the receiver. A low SNR leads to lossof data.

[0007] A prior art CDMA communication system is shown in FIG. 1. Thecommunication system has a plurality of base stations 20 ₁, 20 ₂ . . .20 _(N) connected together through a local Public Switched TelephoneNetwork (PSTN) exchange. Each base station 20 ₁, 20 ₂ . . . 20 _(N)communicates using spread spectrum CDMA with mobile and fixed subscriberunits 22 ₁, 22 ₂ . . . 22 _(N) within its cellular area.

[0008] Shown in FIG. 2 is a simplified CDMA transmitter 24 and receiver26. A data signal having a given bandwidth is mixed with a spreadingcode generated by a pseudo random chip code sequence generator producinga digital spread spectrum signal for transmission. Upon reception, thedata is reproduced after correlation with the same pseudo random chipcode sequence used to transmit the data. By using different pseudorandom chip code sequences, many data signals or subchannels can sharethe same channel bandwidth. In particular, a base station 20 ₁ cancommunicate with a group of subscriber units 22 ₁, 22 ₂ . . . 22 _(N)using the same bandwidth. Forward link communications are from the basestation 20 ₁ to the subscriber unit 22 ₁, 22 ₂ . . . 22 _(N), andreverse link communications are from the subscriber unit 22 ₁, 22 ₂ . .. 22 _(N) to the base station 20 ₁.

[0009] For timing synchronization with a receiver 26, an unmodulatedpilot signal is used. The pilot signal allows respective receivers 26 tosynchronize with a given transmitter 24, allowing despreading of atraffic signal at the receiver 26. In a typical CDMA system, each basestation 20 ₁, 20 ₂ . . . 20 _(N) sends a unique global pilot signalreceived by all subscriber units 22 ₁, 22 ₂ . . . 22 _(N) withincommunicating range to synchronize forward link transmissions.Conversely, in some CDMA systems for example in the B-CDMA™ airinterface each subscriber unit 22 ₁, 22 ₂ . . . 22 _(N) transmits aunique assigned pilot signal to synchronize reverse link transmissions.

[0010]FIG. 3 is an example of a prior art transmitter 24. Data signals28 ₁, 28 ₂ . . . 28 _(N) including traffic, pilot and maintenancesignals are spread using respective mixers 30 ₁, 30 ₂ . . . 30 _(N) withunique chip code sequences 32 ₁, 32 ₂ . . . 32 _(N), respectively. Eachmixers' output is coupled to a combiner 34 which adds the individualmixed signals as a combined signal 44. The combined signal 44 ismodulated up to radio frequency (RF) by a mixer 36 mixing the combinedsignal 44 with an RF carrier, shown in FIG. 3 as COS ωt. The modulatedsignal is amplified to a predetermined transmission power level (TLP) byan amplifier 38 and radiated by an antenna 40.

[0011] Most CDMA systems use some form of adaptive power control. In aCDMA system, many signals share the same bandwidth. When a subscriberunit 22 ₁, 22 ₂ . . . 22 _(N) or base station 20 ₁, 20 ₂ . . . 20 _(N)is receiving a specific signal, all the other signals within the samebandwidth are noiselike in relation to the specific signal. Increasingthe power level of one signal degrades all other signals within the samebandwidth. However, reducing TLP too far results in undesirable SNRs atthe receivers 26. To maintain a desired SNR at the minimum transmissionpower level, adaptive power control is used.

[0012] Typically, a transmitter 24 will send a signal to a particularreceiver 26. Upon reception, the SNR is determined. The determined SNRis compared to a desired SNR. Based on the comparison, a signal is sentin the reverse link to the transmitter 24, either increasing ordecreasing transmit power. This is known as forward channel powercontrol. Conversely, power control from the subscriber unit 22, to thebase station 20, is known as reverse channel power control.

[0013] Amplifiers 64 ₁, 64 ₂ . . . 64 _(n) are used for adaptive powercontrol in FIG. 3. The amplifiers 64 ₁, 64 ₂ . . . 64 _(n) are coupledto the inputs of the combiner 34 to individually control each signal'spower level.

[0014]FIG. 4a, 4 b, 4 c and 4 d show a simplified illustration of threespread spectrum signals 42 ₁, 42 ₂, 42 ₃ and a resultant combined signal44. Although each signal 42 ₁, 42 ₂, 42 ₃ is spread with a differentpseudo random chip code sequence, each signal 42 ₁, 42 ₂, 42 ₃ issynchronous at the chipping rate. When the individual chips within thesequences are summed, the combined signal may have extreme transients46, 48 where the chip energies combine or low transients 47 where theysubtract.

[0015] High transient peaks are undesirable. For every 3 dB peakincrease, twice the base amplification power in Watts is required. Notonly does the transient burden the amplifier, but the power sourcing theamplifier must have a capacity greater than the maximum transient thatmay be expected. This is particularly undesirable in hand-held batteryoperated devices. Additionally, to design for higher power levelsresulting from high transients, more complex amplifier circuitry isrequired or compromises between amplifier gain, battery life andcommunication time result. High valued transients force the amplifier 38into the nonlinear region of its dynamic range resulting in increasedout-of-band emissions and reduced amplifier efficiency. Accordingly,there exists a need for an adaptive RF transmitter system that addressesthe problems associated with the prior art.

SUMMARY OF THE INVENTION

[0016] The invention reduces transient peaks in signals transmitted inCDMA communication systems. A plurality of spread spectrum data signalsare combined into a combined signal having fluctuating power levelcorresponding to the combination of the data signals. The combinedsignal is modulated to produce an RF signal for transmission. Theaverage power of the combined signal is measured over a selected timeperiod. The combined signal power level is adaptively limited to acalculated power level based at least in part on the measured power.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is an illustration of a prior art CDMA system.

[0018]FIG. 2 is an illustration of a prior art CDMA transmitter andreceiver.

[0019]FIG. 3 is a system block diagram of a prior art transmitter.

[0020]FIG. 4a is an illustration of a first pseudo random chip codesequence.

[0021]FIG. 4b is an illustration of a second pseudo random chip codesequence.

[0022]FIG. 4c is an illustration of a third pseudo random chip codesequence.

[0023]FIG. 4d is an illustration of the combined chip code sequences ofFIGS. 4a-4 c.

[0024]FIG. 5 is a system block diagram of an embodiment of the inventionwith the power measurement device coupled to the amplifier.

[0025]FIG. 6 is a system block diagram of an alternate embodiment of theinvention with the power measurement device coupled to the modulator.

[0026]FIG. 7 is an illustration of the probability distribution functionof the power levels of a combined signal.

[0027]FIG. 8 is a plot of the loss in the received signal to noise ratioversus the clipping level.

[0028]FIG. 9 is a plot of the loss in the received signal to noise ratioversus the clipping level in a CDMA communication system using adaptivepower control.

[0029]FIG. 10 is a system block diagram of an alternate embodiment ofthe invention with the processor controlling the amplifier gain.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The preferred embodiments will be described with reference to thedrawing figures where like numerals represent like elements throughout.

[0031]FIGS. 5 and 6 depict transmitter systems of the invention. A groupof data signals 28 ₁, 28 ₂ . . . 28 _(N) that include traffic, pilot andmaintenance signals are mixed with different chip code sequences 32 ₁,32 ₂ . . . 32 _(N) and are summed together in a combiner 34 as acombined signal 44. The combiner 34 is coupled to an adjustable signallimiter 50 (clipper) where signal power levels are hard limited to +βand −β dB. Power levels in between +β and −β are not affected. Thelimited signal 45 is modulated up to RF by a mixer 36. The modulatedsignal is amplified by an amplifier 38 to a predetermined power leveland radiated by antenna 40.

[0032]FIG. 7 illustrates a typical probability distribution function ofthe combined signal power level. Combined chip sequences 46, 47, 48 asshown in FIG. 4d will have an associated power level. The probability ofgiven combined chip sequences having a particular power level is shownin FIG. 7. The two extreme power levels are +K and −K. As shown in FIG.7, the probability of a given combined sequences chip having a powerlevel of +K or −K is extremely low. Whereas, the probability of combinedchip sequences having a power level in the middle of the two extremes ishigh. Since a spread spectrum signal is spread across a widecommunication bandwidth and there is a low probability that combinedchip sequences will have a power level at the ends of the distribution,the combined signal 44 can be clipped below these extremes withinsignificant loss.

[0033] The transmitter system adjusts the clipping levels, β, toeliminate the signal transients with only a small decrease in thetransmittal signal-to-noise ratio (SNR). FIG. 8 is a graph illustratingthe relationship between SNR and clipping levels for a system not usingadaptive power control. The solid line, dashed line and dotted line,respectively, depict communication channels with different operatingSNRs. As shown in FIG. 8, for a β set at a clipping level of twostandard deviations the loss in SNR is negligible and at a clippinglevel of one standard deviation the loss is only approximately 0.2 dB.

[0034] For a system using adaptive power control, FIG. 9 is a graph ofSNR versus the clipping level. The results are similar to those obtainedin a system not using adaptive power control. As shown in FIG. 9, with aclipping level of two standard deviations, the loss in SNR is againnegligible. Accordingly, the clipping circuitry is applicable to systemsutilizing adaptive power control and systems not using adaptive powercontrol.

[0035] Referring back to FIG. 5, to determine β, the invention uses apower measurement device 52 and a processor 54. The power measurementdevice 52 is coupled to either the output of the RF amplifier 38 asshown in FIG. 5 or the mixer 36 as shown in FIG. 6. Preferably, thepower measurement device 52 determines the average of the square of themagnitude of the transmitted signal over a predetermined time period.The output of the preferred power measurement device 52 approximates thevariance of the mixed signal 49 or the signal 51 being transmitted.Alternatively, the power measurement device 52 determines anapproximation of the standard deviation by taking the average of theabsolute value of the signal 49, 51 or the power measurement device 52measures the magnitude of the signal 49, 51 with the processordetermining either the variance or standard deviation.

[0036] The output of the power measurement device 52 is coupled to aprocessor 54. If the power measurement device 52 is coupled to theoutput of the amplifier 38, the processor 54 scales down the output ofthe power measurement device 52 by the gain of the amplifier 38. Theprocessor 54 determines the proper clipping level for β. Depending onthe desired SNR and bandwidth, the value for β will be a multiple of thestandard deviation. If the power measurement device 52 approximates thevariance, the processor 54 will take the square root of the device'soutput as the standard deviation. In the preferred embodiment, β will betwo times the standard deviation.

[0037] In certain situations, the processor 54 overrides the determinedvalue of β. For instance, if the transmitter 25 was used in a basestation 20 ₁, 20 ₂ . . . 20 _(N), a large increase in the number ofusers may result in β being temporarily set too low. This will result inan undesirable received SNR. As supplied to the processor 54 through theline 60, the number of users currently in communication with the basestation 20 ₁, 20 ₂ . . . 20 _(N), is used to either change β ortemporarily disable the clipper 50 to allow all signals to passunaltered when appropriate.

[0038] Additionally, since the probability distribution function assumesa large sample size, a small number of users may result in an undesiredreceived SNR. Accordingly, if only a few users were in communicationwith the base station 20 ₁, 20 ₂ . . . 20 _(N), the clipper 50 may bedisabled. In addition, when there are only a small number of usersactive, the amplifier's dynamic range is not reached. Accordingly, thereis no need to clip the combined signal. Under other situations, it maybe necessary to override the clipper 50. For instance, in some CDMAsystems short codes are used during initial power ramp up. Since thesecodes are not long enough to approximate a random signal, by chance onecode may result in a large number of high transient peaks within thesignal. Clipping these transmissions may dramatically decrease thereceived SNR and unnecessarily delay the initial power ramp upprocedure. In these situations, a signal will be sent to the processor54 through the line 62 to override the clipper 50.

[0039] In an alternate embodiment shown in FIG. 10, the processor 54 isalso used to control the gain of the amplifier 38 through the line 58.Stored in the processor is the amplifier gain characteristic. Theamplifier gain is adjusted to keep the amplifier from going into thenonlinear operating region. Accordingly, out-of-band emissions andinterference to services in adjoining frequency bands is reduced.

[0040] Although the invention has been described in part by makingdetailed reference to certain specific embodiments, such detail isintended to be instructive rather that restrictive. It will beappreciated by those skilled in the art that many variations may be madein the structure and mode of operation without departing from the scopeof the invention as disclosed in the teachings herein.

What is claimed is:
 1. A user equipment (UE) for transmitting signalsemploying a CDMA technique, comprising: means for combining a pluralityof spread spectrum data signals; means for measuring a characteristic ofthe output of said combining means for a given time period; and meansfor adaptively limiting an output of the combining means responsive atleast partially to an output of said measuring means.
 2. The UE of claim1 further comprising means for modulating the output of said combiningmeans to produce an RF signal, wherein said measuring means measures anoutput of the modulating means over the given time period.
 3. The UE ofclaim 2 further comprising an amplifier for amplifying the RF signal,and wherein said measuring means measures an output of the amplifierover the given time period.
 4. The UE of claim 1 wherein said measuringmeans determines a variance of the output of said combining means, andwherein said adaptive limiting means limits the output to a given powerlevel based in part on an approximation of the variance.
 5. The UE ofclaim 1 wherein said measuring means determines an average of a squareof said output, and wherein said adaptive limiting means limits theoutput to a given power level based in part on the average of the squareof said output.
 6. The UE of claim 1 wherein said measuring meansdetermines an average of an absolute value of said output; and saidadaptive limiting means limits said output to a given power level basedin part on the average of the absolute value of said output.
 7. The UEof claim 1 wherein said measuring means determines the magnitude of saidoutput, said measuring means having processing means for determining avariance of said output based on the determined magnitude of saidoutput; and wherein said adaptive limiting means limits said output to acalculated power level based in part on the determined variance.
 8. TheUE of claim 1 wherein said measuring means has processing means fordetermining a standard deviation of said output and said adaptivelimiting means limits said output to a given power level based in parton the determined standard deviation.
 9. The UE of claim 8 wherein thecalculated power level is twice as great as a standard deviation. 10.The UE of claim 8 wherein the calculated power level is at least onestandard deviation.
 11. The UE of claim 8 wherein said processing meandisables said adaptive limiting means when a number of active usersreaches a given value.
 12. The UE of claim 8 wherein said processingmeans disables said adaptive limiting means during transmission of shortcodes.
 13. The UE of claim 1 further comprising a modulator formodulating said output to produce an RF signal and an amplifier foramplifying the RF signal prior to transmission, said measuring meansdetermining a power level of said output; wherein a gain of saidamplifier is adjusted by a processing means in response to thecalculated power level and stored gain characteristics of saidamplifier.